Solved: The Mystery of the Martian Meteorites

Planetary scientists studying Mars have a slightly embarrassing secret: They don’t really know how old most of the planet’s surface is. They do have decent estimates, mostly based on counting craters pockmarking the Martian crust—more craters equate to a greater age. Yet the only way to pin down an age with something approaching absolute certainty is to closely analyze rock samples, and none of the rovers and landers set down on the Red Planet has carried the necessary equipment. Without precise ages the entire history of the planet is blurred, making it more difficult to answer important questions about when and whether Mars was ever truly habitable.

Fortunately, there are Martian rocks right here on Earth. Asteroids or comets can hit Mars hard enough to hurl chipped-off fragments of crust on interplanetary voyages to our world. Some 120 specimens out of the more than 60,000 meteorites in collections around the globe contain mixtures of minerals and microscopic air bubbles that match what we know of the Martian surface and atmosphere. Researchers can date these rare samples by measuring certain radioactive isotopes within them, because the isotopes decay into other elements at rates set by the laws of physics. With most igneous rocks, which begin life as molten material, calculating the ratio of a long-lived isotope, such as uranium 238, to its decay product, lead 206, yields a very good estimate of just how old that rock is—how long ago its isotopes became locked in minerals crystallizing out from a molten mass.

The trouble is that different isotopic tracers yield wildly different dates for the most common variety of Martian meteorites, hunks of igneous rock called shergottites. Grind up a whole shergottite, and the ratio of lead isotopes in the powder will suggest the rock is about four billion years old. If you instead look at various isotopes isolated within microscopic mineral grains inside the shergottite, you will conclude the rock is relatively youthful—only hundreds of millions of years old. This conundrum has flummoxed researchers for years, leaving them divided about the timing and duration of Martian volcanic activity, or when the consolidation of the Martian core and mantle occurred. Now, however, the matter seems to be settled: In a report appearing in the July 25 edition of Nature, a team of scientists lead by Desmond Moser of the University of Western Ontario has presented substantial new evidence that shergottites are young. They base their conclusions on a half-kilogram fragment of Mars known only as Northwest Africa 5298 (NWA 5298). (Scientific American is part of Nature Publishing Group)

“Lots of groups just jump to [isotope] dating right away,” says Kim Tait, a study co-author and mineralogist at the Royal Ontario Museum (ROM) in Toronto, which provided the sample of NWA 5298. “For us, the dating came absolutely last. We first looked very carefully at the minerals, scanning grain by grain so that we could really understand everything in context.... Every rock has a story to tell. Interpreting clues to uncover that story is where we come in.”

“These tiny meteorites are packed with stories on the evolution of a whole planet, stories that we can’t currently get from rovers,” Moser says. “What we’ve done is sort out the ‘page numbers’ for the stories preserved in these rare fragments from space.”

According to Tony Irving, a University of Washington geochemist who first identified NWA 5298’s Martian provenance, a desert nomad discovered the meteorite in 2008 outside the Moroccan village of Bir Gandouz. An anonymous Moroccan middleman purchased the rock from the nomad and eventually sold it for an undisclosed sum to David Gregory, a Canadian physician who later donated it to the ROM. Analyzing a small sample of the meteorite, Irving recognized it as a shergottite based on its characteristic chemical composition as well as its network of “shock metamorphosed” glassy veins and vesicles formed by the immense pressure of an ejecting impact. He also noted the presence of micron-scale grains of baddeleyite, a highly durable zirconium-rich mineral that is often used in uranium–lead dating.

Those baddeleyite grains proved to be keystones for Moser’s subsequent investigations, which found them to be only hundreds of millions of years old. This was consistent with previous studies, but proponents of the idea that shergottites are billions of years old have long argued that the youthful baddeleyite grains were generated by the great heat and stress of an ejecting impact and were thus not representative of the rock’s true age.

Using a sophisticated scanning electron microscope, Moser and his colleagues first mapped the baddeleyite grains within a thin cross section of NWA 5298, then closely studied each one for clues about its past. Many of the grains bore concentric bands of material indicative of gradual growth, which in turn suggested they formed in slow-cooling magma. Subsequent uranium–lead radiogenic isotope dating showed their age to be approximately 187 million years. Even closer examination revealed that the grains’ original crystalline structure had been obliterated by the passage of a sudden, strong shock wave. The only remaining crystalline minerals, in fact, were vanishingly thin rims of silica-rich zircon that had flowed around and flash-frozen onto the baddeleyite grains—a sign of shock melting followed by cooling in the frigid vacuum of space. Uranium and lead isotopes extracted from grains with those rims yielded an estimated age of less than 22 million years. After correcting for sources of lead contamination originating on Earth, such as the burning of leaded gasoline, measurements of lead isotopes from the bulk mineral matrix around the baddeleyite grains yielded a date in excess of four billion years. That bulk mineral matrix also showed a strange preponderance of inert Martian lead, a sign that the region deep within Mars where the rock first formed had stopped mixing with its surroundings a long time ago.

“What this means is that both the young and old age estimates are ‘right,’ but they mean different things,” says Irving, who was not involved in the study. The young baddeleyite formed nearly 200 million years ago when the shergottites’ source rock crystallized on or near the Martian surface. The far older lead, he says, is an artifact from a time about four billion years ago when most of the deep Martian interior had already cooled and ceased convecting.

As for why all that lead was there in the first place, Moser and his colleagues suggest its presence may be related to one or more truly giant impacts more than four billion years ago that struck with such disruptive force they permanently altered the chemistry of the Martian interior. Tentative evidence for such massive impacts can be seen in hemisphere-spanning differences in elevation and crustal thickness on Mars.

Thus, from a single, small meteorite, a rather epic narrative suggests itself: Before four billion years ago Mars was already dying, rapidly losing its interior heat to space, when a planetoid leftover from the solar system’s formation may have slammed into it, melting some of the surface and sweeping away a significant portion of the atmosphere. The great scar from the impact gradually froze over and healed, and Mars settled into senescence. Much later, some 187 years million years ago, as dinosaurs walked the Earth, a slowly upwelling magma plume breached the Martian surface, pouring out young lavas imprinted with the signature of the ancient impact; these slowly crystallized and formed the baddeleyites. At least another 165 million years passed without incident, until a mountain-size asteroid fell out of the sky. The impact launched partially melted fragments of the crystallized lava up through the atmosphere and into space, glossing the baddeleyite grains with rims of molten zircon. One of those fragments drifted and spun alone in the dark for millions of years more, until some tens of millennia ago it crossed paths with a neighboring planet, falling to Earth in a fireball that landed in what is now the western Sahara desert of southern Morocco. It languished there, just another brown weathered rock in the sand, until, by chance, a passing person picked it up in 2008.

None of this dramatically revises our understanding of Mars, but it does finally put better dates on some key events in the planet’s history while also raising new questions about its deep interior. The most noteworthy aspect of this abstruse dating study, though, is that its insights suggest ways to unravel other mysteries throughout the solar system.

What we’ve done in this study can be applied to any rock, anywhere,” Tait says. “From primitive meteorites to asteroids to the inner rocky planets, we can use these analytical techniques to start piecing together a more detailed and accurate history of the solar system.”